The supercritical carbon dioxide (hereinafter, may be expressed as scCO2) refers to a fluid of carbon dioxide formed at the critical temperature (31.1° C.) or higher and under the critical pressure (73.8 bar) or higher, and has characteristics relatively similar to those of a nonpolar solvent such as hexane, so that scCO2 is expected as an environmentally acceptable industrial solvent. For example, a thermodynamically stable W/scCO2 microemulsion (hereinafter, may be expressed as W/scCO2μE) in which water (W) or the like is dispersed as nanometer-seized aqueous droplets in scCO2 is expected to be applicable to various fields such as dry-cleaning, extraction of useful components such as metal ions and proteins, and an organic synthesis or a fine particle synthesis by utilization thereof as a reaction field.
For forming W/scCO2μE, a surfactant soluble in scCO2 is necessary, so that hitherto, various surfactants have been studied. Among them, it is known that a fluorocarbon compound can be well dissolved in scCO2 (Patent Document 1). However, the fluorocarbon compound is expensive and it is necessary to avoid an excessive use thereof also in terms of the burden to the human body and the environment. Therefore, the development of a hydrocarbon-based surfactant is desired.
As a representative example of a hydrocarbon-based surfactant, AOT (Aerosol-OT [registered trade mark]: sodium bis-2-ethyl-1-hexylsulfosuccinate) is known. However, this surfactant is not suitable as a surfactant for W/scCO2μE, and the surfactant is entirely not dissolved in scCO2 or even when the surfactant can form a microemulsion, W0c is a maximum of 10 or less (Non-patent Document 1, p. 12, 1.35-41). Here, W0c is a value remaining after subtracting an amount (a number of moles) of water capable of being dissolved in carbon dioxide from a molar ratio (W0) of water relative to 1 mole of a surfactant, and is an index for the ability of a surfactant to form a microemulsion in scCO2 (ability of dispersing water). As an effective surfactant for W/scCO2μE, TMN-6 (Tergitol [registered trade mark]: polyethylene glycol trimethyl nonyl ether) is known (Non-patent Document 2, p. 112, 1.19-29 or Non-patent Document 3). However, it is a present situation that except the TMN-6, there is reported no effective surfactant among other commercially available hydrocarbon-based surfactants.